Production of alkali metal hydroxides and chlorine by electrolysis of brines is well known in the art. There are many known cell systems employed for this purpose and most of these systems have some common characteristics. Thus, generally, the electrolysis cells employed for the production of caustic and chlorine consist of compartments, designated as anode and cathode compartments. The anode compartment serves for the electrolytic decomposition of aqueous brine, such as an NaCl solution according to equation (1) below: EQU 2Na.sup.+ + 2Cl.sup.- -2E 2Na.sup.+ + Cl.sub.2 ( 1)
while in the cathode compartment, electrolysis of water takes place in accordance with equation (2) below: EQU 2H.sub.2 O .sup.+2E 2OH.sup.- + H.sub.2 ( 2)
the sodium ions from the anolyte compartment combine with the hydroxyl ions generated in the catholyte compartment resulting in the formation of an aqueous sodium hydroxide solution as shown in equation (3): EQU Na.sup.+ + OH.sup.- .fwdarw. NaOH (3)
for many years, porous diaphragms were used to separate the anode and cathode compartments. The diaphragms served to separate the cell gaseous products and permitted brine to flow from the anode compartment to the cathode compartment. The brine transport across the diaphragm provided an electrical path for the migration of sodium ions to the cathode compartment. The caustic product formed in the cathode compartment was limited to a concentration of 12% and was contaminated with salt resulting from the brine flow across the diaphragm. The weak caustic product was then concentrated by crystallizing evaporation to a commercial grade containing 50% caustic, contaminated with 1% salt.
In recent years, membrane cells have been developed, in which the porous diaphragm has been replaced with a membrane material. This permits the transport of sodium ions from the anode compartment to the cathode compartment, but prevents transfer of the brine solution. This development has provided a means for production of a high concentration salt-free caustic.
The use of membrane-equipped electrolytic cells has not become widespread in the past because of problems encountered with the stability of the membranes and with the relatively low current and power efficiencies in comparison to the conventional diaphragm cell. Recent efforts have resulted in the development of significantly improved membranes which focused new attention on membrane-equipped electrolytic cells for the production of caustic by brine electrolysis. Thus, it has been recently announced by Asahi Chemical Industry Co., Ltd., in a presentation to The American Chemical Society, during the Centennial meeting at New York, held on Apr. 4-9, 1976, that the world's first membrane-equipped commercial chlor-alkali plant with an annual production capacity of 40,000 metric tons has been put in operation in 1975. The membrane employed in the Asahi Chemical system, as reported at the abovereferenced presentation, is a perfluoro carboxylic acid membrane which in comparison to previously recommended perfluoro sulfonic acid membranes, provides stable operations at high current efficiency.
Key economic factors in the operation of membrane cells are the achievement of a long membrane life, the attainment of a high current efficiency, the realization of a low voltage drop at high current densities and the production of a high concentration caustic at the cell. The degree of achievement of these factors is dependent on the physical characteristics of the specific membrane material and will vary with operating conditions.
Maintenance of a high current density reduces the original plant capital investment and the membrane replacement costs. Similarly, a high product concentration reduces the capital and operating costs of evaporation equipment required to bring the caustic product up to commercial concentration. Low voltage drop and high current efficiency reduce the energy requirements which are a substantial portion of the total manufacturing cost. The high cost of membrane materials and maintenance costs associated with membrane replacement require achievement of a long membrane life.
Considerable membrane development work has been done using, primarily, various formulations of perfluoro sulfonic acid and perfluoro carboxylic acid type resins. Each of these membrane formulations has specific properties which vary with the operating conditions. A particular membrane material may have excellent current efficiency characteristics at a 40% caustic concentration, but may have a short life under these conditions. As a result, the cell operating conditions and commercial membrane materials in use today represent a compromise aimed at attainment of the best achievable product cost.
The economics of a typical membrane cell installation show that energy requirements account for 50% of the total manufacturing cost. Capital investment and membrane replacement amount to 25% and 8%, respectively, of the total manufacturing cost.
From these figures, it can be seen that production of caustic and chlorine by membrane cell electrolysis is a relatively energy and capital intensive process. Modifications to the electrolysis process resulting in energy and capital savings, are therefore of foremost importance.
It has now been found that significant savings can be achieved by using the instant system for the production of caustic, wherein in at least one cell of a membrane-equipped electrolysis system, a caustic solution of relatively low concentration is produced by electrolysis of NaCl containing brine. Caustic of desired high concentration is then produced by electrolyzing brine in a second stage using the dilute caustic produced in the first stage in lieu of water. This staging of the caustic production results in lower energy consumption/ton of caustic produced in comparion to prior art systems with simultaneous increase in the average life of the membranes employed and reduction in capital investment.